Multifunctional monomers are customarily incorporated in polymers at low levels in order to provide for crosslinking of the polymer. At high levels, however, polymerization of multifunctional monomers characteristically proceeds only to low conversions due to trapping of residual reactive functionality within the rigid, crosslinked network as it is formed. Preparation of epoxy functional and 1-propenyl ether functional methyl siloxanes has been described by applicant. These oligomers undergo rapid thermally induced and photoinduced cationic polymerization and proceed to high conversions. The anomalous behavior in the case of the siloxane-containing monomers has been attributed to the conformational flexibility of the siloxane (Si--O--Si) bond and to free volume effects.
Preparation of similar multifunctional alkoxy siloxanes is difficult to accomplish by prior art methods. Epoxy-functional trialkoxy silanes employed as glass coupling agents to increase adhesion between epoxy matrices and reinforcing glass fibers are commercially available as starting materials. However, condensation polymerization of multifunctional alkoxy silanes generally results in a crosslinked gel.
Sol-gel techniques have been widely employed for the polymerization of alkoxy silanes, resulting in crosslinked glass-like matrices composed of Si--O--Si linkages. Typically, alkoxy silanes such as tetraethoxysilane (TEOS) are subjected to acid or base catalyzed hydrolysis-condensation in the presence of controlled amounts of water to yield a gel. In most acid catalyzed sol-gel processes, HCl is used, while NaOH and NH.sub.4 OH are often employed as base catalysts.
However, while conventional sol-gel chemistry is convenient and easily carried out, it suffers from drawbacks which make it less attractive for the preparation of stable multifunctional oligomers. When sol-gel condensations are carried to completion, crosslinked matrices result. It is difficult to control the sol-gel reaction so that one can reproducibly make intermediate, soluble, low viscosity, fluid oligomers. When such materials are obtained, they exhibit poor pot-lives and gel on standing due to further condensation.
In addition, the basic hydrolysis catalysts used in the sol-gel reaction are strong inhibitors for cationic polymerizations, and it is difficult or impossible to remove conventional basic catalysts at the end of the reaction. Further, acid hydrolysis catalysts are not useful for the synthesis of epoxy-functional siloxanes since epoxy groups undergo spontaneous ring-opening reactions with acids.
There is therefore a need for a process for making pure multifunctional alkoxy siloxane oligomers reproducibly and in good yield. There is also a need for alkoxy siloxane oligomers that have long pot-lives and low viscosity.